The Long Term Evolution (LTE) Release 12 (Rel-12) standard under consideration by the Third Generation Partnership Project (3GPP) provides support for device-to-device (D2D) communications targeting both commercial and public safety applications. Devices configured for D2D communications can communicate directly with one another without an intervening base station. One application for D2D communications is direct vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), and vehicle-to-pedestrian (V2P) communications. D2D communications involving at least one vehicle are referred to generally as vehicle-to-device (V2x) communications. V2x communications may take advantage of network infrastructure when available, but direct V2x communications should be possible when no network is available.
V2x communications have been proposed for road safety applications, in addition to general communications. For instance, the European Telecommunications Standards Institute (ETSI) has defined two types of messages for road safety applications: the Co-operative Awareness Message (CAM) and the Decentralized Environment Notification Message (DENM). CAMs enable emergency vehicles or other vehicles to broadcast warning messages to notify other devices of its presence. CAMs can also be used to improve safety under normal traffic conditions. DENMs are event-triggered messages, which may be triggered by braking, turning, acceleration, deceleration, or similar events. CAMs and DENMs may target vehicles, pedestrians, and/or infrastructure. Under the ETSI standard, a device may be configured to check for CAMs and/or DENMs every 100 ms. The detection latency for CAMs and DENMs is 100 ms, which is longer than the 50 ms detection latency required for pre-crash sensing. The package size of CAMs and DENMs varies from about 100 to more than 800 bytes and the typical size is about 300 bytes. Ideally, CAMs and DENMs should be detected by all vehicles in proximity to the vehicle broadcasting the message.
Besides the ETSI, other regulating organizations have also provided standards defining D2D messages aimed at improving road safety. For example, the Society of Automotive Engineers (SAE) has also defined a message for V2x communications known as the Basic Safety Message (BSM). BSMs may be classified into different priority levels based on the importance or urgency of the message.
In D2D networks, and especially V2x networks, devices typically share a pool of radio resources that are accessed on a contention basis, with limited supervision or control by the network. A distributed resource allocation scheme is used where devices select resources for transmissions from a shared resource pool on a contention basis after a sensing operation of the radio environment. Ideally, the distributed resource allocation converges to a stable equilibrium point that efficiently utilizes the radio resources.
One concern with distributed resource allocation schemes, particularly for V2x communications, is that the radio conditions are constantly changing and that devices must reselect radio resources as the radio conditions change. The selection or reselection of radio resources by one device may affect the selection or reselection of radio resources by other devices. In this case, the distributed resource allocation may not converge to a stable solution even in this absence of mobility. Unstable resource allocation and can lead to degraded performance and inefficient use of the radio resources.
Another concern with distributed resource allocation is that many wireless devices operate in a half-duplex mode, which means that the wireless devices cannot transmit and receive at the same time. Two devices in close proximity may experience similar radio conditions and thus select the same radio resources for transmissions, which is sometimes referred to as a collision. If two devices operating in half-duplex mode select the same radio resources for transmission, they will not be able to communicate with each other. This problem may be mitigated by randomizing resource selection at each device to minimize the likelihood of selecting the same radio resources. However, randomizing the resource selection may prevent the distributed resource allocation from converging to a stable solution.